Succession in a Tropical Dry Forest: A Test of the Chronosequence and Inference of Community Assembly Dynamics

To better understand successional changes and drivers of changes in forests, we must (1) test assumptions of successional studies and (2) quantify patterns in species and functional traits in stands varying in successional age. Successional changes are commonly approximated by quantifying changes along chronosequences, or space-for-time substitutions. These assumed successional patterns, however, are rarely tested through longitudinal studies of individual sites. To identify drivers of successional trajectories and to address questions of deterministic vs. stochastic community assembly, species composition data must be supplemented with data such as species functional traits. We examined patterns in species composition and functional traits along a chronosequence of tropical dry forest (TDF) sites in North Key Largo, Florida in 1993. To test patterns identified along the chronosequence, we quantified longitudinal change in species and functional traits by remeasuring the sites in 2013. We tested the following predictions: (1) dynamic changes within sites in species composition and functional trait composition will corroborate a pattern indicated by the chronosequence; (2) a growth-survival demographic tradeoff will be consistent with increasing dominance of evergreen species over the course of succession; and (3) forest succession will demonstrate convergence in both composition and function. As predicted, dynamic changes in species composition and functional traits within sites corroborated patterns quantified over the chronosequence, and species functional groups showed a growth-survival demographic tradeoff between deciduous species early in succession and evergreen species late in succession. Surprisingly, however, although species composition converged late in succession, functional traits showed greatest divergence late in succession. We suggest that this divergence is due to functionally disparate evergreen and leaf exchanger species coexisting with high abundances for the first time late in succession, and to interspecific competition in an environment of limited resources late in succession. Synthesis. This is the first study in TDF to corroborate compositional and functional changes measured along a chronosequence with results of longitudinal measurements within sites. Additionally, this TDF ecosystem shows a rare successional pattern of convergence in species composition but divergence in functional traits, likely due to the coexistence of different functional groups and interspecific competition late in succession.